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RFID Security 623
24.5.1 Tag/Label
RFID units are in a class of radio devices known as transponders . A transponder is a
combination transmitter and receiver, which is designed to receive a specific radio signal and
automatically transmit a reply. In its simplest implementation, the transponder listens for a radio
beacon, and sends a beacon of its own as a reply. More complicated systems may transmit a
single letter or digit back to the source, or send multiple strings of letters and numbers. Finally,
advanced systems may do a calculation or verification process and include encrypted radio
transmissions to prevent eavesdroppers from obtaining the information being transmitted.
Transponders used in RFID are commonly called tags , chips , or labels , which are fairly
interchangeable, although “ chip ” implies a smaller unit, and “ tag ” is used for larger
devices. The designator label is mainly used for the labels that contain an RFID device.
(The term “ tag ” is used for the purposes of this book.)
As a general rule, an RFID tag contains the following items:
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Encoding/decoding circuitry
Memory
Antenna
Power supply
Communications control
Tags fall into two categories: active and passive (see Figure 24.4 ).
24.5.1.1 Passive vs. Active Tags
Passive RFID tags do not contain a battery or other power source; therefore, they must
wait for a signal from a reader. The tag contains a resonant circuit capable of absorbing
power from the reader’s antenna. Obtaining power from the reader device is done using
an electromagnetic property known as the Near Field . As the name implies, the device
must be relatively near the reader in order to work. The Near Field briefly supplies
enough power to the tag so that it can send a response.
In order for passive tags to work, the antenna and the tag must be in close proximity
to the reader, because the tags do not have an internal power source, and derive their
power to transmit from coupling to the Near Field of the antenna. The Near Field takes
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